deviation from the normal structure of an organism that occurs during intrauterine or, less commonly, postpartum development. Developmental anomalies, which are different from extreme abnormalities, are caused by a variety of internal (heredity, hormonal disturbances, biological inferiority of sex cells) and external (ionizing radiation, viral infection, oxygen deficiency, exposure to certain chemicals) factors.

Developmental anomalies have become increasingly common since 1950, especially in the economically developed countries. The mechanism of these anomalies is complex and has not been adequately studied. Experimental embryologists have demonstrated that the teratogenetic period, that is, the interval during which a teratogenic agent can cause a congenital anomaly, is a major factor in determining the production of the defect. This period varies from organ to organ. Using the findings of embryo-logical research, it is possible to estimate the time when a given anomaly arises and to prepare a teratological calendar for anomalies of different organs.

Anomalies may also arise owing to the cessation of development in a critical period, interference with the formative process (dysontogenesis), and the destruction of tissue. Organs or parts of organs may not develop fully (hypogenesis) or may develop excessively (hypergenesis). Some organs or parts of the body may be absent (agenesis), the organs may be in an incorrect position or may shift, and some tissue may form abnormally (dysplasia).

A distinction is made between multiple anomalies, resulting from the imperfect development of two or more fetuses, and single anomalies, which are caused by defective morphogenesis in a single organism. A double anomaly, or monster, consists of conjoined twins, which, depending on the site of junction, may constitute a thoracopagus, xiphopagus, or pygopagus. Single developmental anomalies include acrania, harelip, cleft palate, polydactyly, and congenital heart disease. Developmental anomalies may be prevented through good prenatal care.